1. Field
Example embodiments may relate to a laser chip and/or a vertical external cavity surface emitting laser (VECSEL) using the same, for example, to a laser chip having improved efficiency and increased modal gain and/or a VECSEL using the laser chip.
2. Description of the Related Art
Vertical external cavity surface emitting lasers (VECSELs) may have increased gain regions by using an external mirror instead of an upper mirror for a vertical cavity surface emitting laser (VCSEL) and may obtain higher output power of several tens of watts (W) or higher.
FIG. 1 is a schematic view illustrating a related art VECSEL 10. As shown in FIG. 1, the VECSEL 10 may include a laser chip 12 on a heat sink 11, a concave folding mirror 15 opposite the laser chip 12, a flat external mirror 18 that may reflect light reflected from the folding mirror 15 back to the folding mirror 15, a birefringent filter 13 between the laser chip 12 and the folding mirror 15 that transmits only light of a specific wavelength, and/or a second harmonic generation (SHG) crystal 14 between the folding mirror 15 and the flat external mirror 18.
The VECSEL 10 may further include a pump laser 16 for providing a pump beam to the laser chip 12 and/or a lens 17 for focusing the pump beam on the laser chip 12. Although FIG. 1 illustrates a VECSEL 10 in which the pump laser 16 may provide the pump beam to the front of the laser chip 12, the pump beam may be provided to the back of the laser chip 12. The laser chip 12 may include an active layer 12c and/or a distributed Bragg reflector (DBR) layer 12b stacked on a substrate 12a. The active layer 12c may have multiple quantum wells having, for example, a resonant periodic gain (RPG) structure, and may be excited by a pump beam, causing the active layer 12c to emit light having specific wavelengths.
If the pump beam emitted from the pump laser 16 is incident upon the laser chip 12, an active layer 12c in the laser chip 12 may be excited, thereby causing the active layer 12c to emit light. Light emitted from the laser chip 12 may pass through the birefringent filter 13, and/or may be reflected by the folding mirror 15 toward the flat external mirror 18. Light may be sequentially reflected by the flat external mirror 18 and the folding mirror 15 and then may be incident on the laser chip 12. A portion of light may be absorbed by the active layer 12c, and any remainder of light may be reflected by the DBR layer 12b. Thus, light may resonate between the DBR layer 12b and the flat external mirror 18. A cavity between the DBR layer 12b and the flat external mirror 18 may be concave and bounded by the folding mirror 15.
The SHG crystal 14 between the folding mirror 15 and the flat external mirror 18 may double the frequency of incident light (that is, it may half its wavelength). Light wavelength-converted by the SHG crystal 14 may be reflected by the flat external mirror 18 and then may pass to the outside through the folding mirror 15. A surface of the flat external mirror 18 may be coated so that both light that is wavelength-converted and light that is not wavelength-converted may be reflected. Alternatively, the folding mirror 15 may be coated so that light that is not wavelength-converted may be reflected with increased reflectivity and light that is wavelength-converted may be transmitted through the mirror with increased transmissivity.
As shown in FIG. 2, a laser chip 12′ may have an anti-reflection coating 12d on the active layer 12c so that a pump beam may reach the active layer 12c with less reflection loss. As shown in FIG. 3, if a pump beam is provided to the rear of the laser chip, a laser chip 20 may include a substrate 21 having an opening in a central part thereof, a heat spreader 22 on the substrate 21, a DBR layer 23 on the heat spreader 22, an active layer 24 on the DBR layer 23, and/or a pump beam reflective layer 25 that may reflect a pump beam from the active layer 24 back to the active layer 24.
In related art laser chips, the phase of light resonating in a cavity may be shifted due to the anti-reflection coating and/or the pump beam reflective layer on an active layer, and utilization of the pump beam may be reduced. A multiple quantum well layer in the active layer may coincide with an antinode of a standing wave in a cavity. If the phase of the standing wave is shifted due to the anti-reflection layer and/or pump beam reflective layer, the location of the anti-node of the standing wave may not coincide with the location of the multiple quantum well layer. As a result, the output of a laser chip which has an anti-reflection coating or the pump beam reflective layer thereon (See graph of FIG. 4B) may be of lesser quality compared with the output property of a laser chip which has no additional layer thereon (See a graph of FIG. 4A).
If an additional layer is on the active layer, an additional gain, which may be caused by a resonating effect in a sub-cavity between an upper surface of the active layer and the DBR layer, may be reduced.